1. Field of the Invention
The present invention relates to a common electrode substrate that is opposed to an array substrate as well as to a liquid crystal display device having the substrate.
2. Description of the Related Art
Liquid crystal display devices have a liquid crystal that is sealed between a pair of substrates. Each of the paired substrates has at least one electrode and an alignment film. TN (twisted nematic) mode liquid crystal display devices, which are widely used conventionally, have horizontal alignment films and a liquid crystal having positive dielectric anisotropy. Liquid crystal molecules are aligned approximately parallel with the horizontal alignment film when a voltage is applied. Liquid crystal molecules rise so as to become approximately perpendicular to the horizontal alignment film when a voltage is applied to those.
While TN mode liquid crystal display devices have such advantages that they can be made thin, they have a first disadvantage of a narrow viewing angle and a second disadvantage of low contrast. A method for solving the first disadvantage and obtaining a wide viewing angle is domain division. In the domain division, each pixel is divided into two domains. In one domain, liquid crystal molecules rise or fall toward one side. In the other domain, liquid crystal molecules rise or fall toward the other side. By forming the domains having different view angle characteristics in each pixel, the view angle characteristic of the device as a whole is averaged, and a wide viewing angle can thereby be obtained.
A usual method for controlling the alignment of a liquid crystal is to rub an alignment film. In the case of the domain division, rubbing is performed in a first direction in one domain of each pixel by using a mask, and then, rubbing is performed in a second direction that is opposite to the first direction in the other domain by using a complementary mask. Alternatively, the entire alignment film is rubbed in a first direction, and then, one domain or the other domain of each pixel is selectively irradiated with ultraviolet light by using a mask to produce a difference in the pre-tilt of liquid crystal molecules between the one domain and the other domain.
Rubbing needs to be performed in liquid crystal display devices having horizontal alignment films. Failures due to pollution and static electricity that occur during the rubbing are factors of lowering the yield.
On the other hand, in VA (vertically aligned) mode liquid crystal display devices having vertical alignment films, liquid crystal molecules are aligned approximately perpendicular to the vertical alignment films when no voltage is applied. Liquid crystal molecules fall so as to become parallel with the vertical alignment films when a voltage is applied. This provides high contrast and solves the second disadvantage (low contrast) of the TN mode liquid crystal display devices. However, even in general VA mode liquid crystal display device having vertical alignment films, the alignment films are usually rubbed to control, the liquid crystal alignment.
Japanese Patent Application No. 185836/1998 of the present applicant proposes a liquid crystal display device in which the liquid crystal alignment can be controlled without rubbing. This liquid crystal display device is a VA mode liquid crystal display device having vertical alignment films and negative dielectric anisotropy. To control the liquid crystal alignment, linear alignment regulating structures (protrusions or slits) are provided on each of paired substrates.
In this specification, this type of VA mode liquid crystal display device will be hereinafter referred to as “MVA (multi-domain vertical alignment) liquid crystal display device.”
The MVA liquid crystal display device has advantages that the rubbing is not necessary and domain division can be attained by arranging linear alignment regulating structures. Therefore, the MVA liquid crystal display device can provide a wide viewing angle and high contrast. Since the rubbing is not necessary, the liquid crystal display device can be manufactured easily and is free of pollution due to dust, etc. that would otherwise be scraped off alignment films during rubbing, leading to an increase of the reliability of the liquid crystal display device.
FIG. 21 is a plan view showing the basic configuration of a conventional MVA liquid crystal display device and shows one pixel and a region in its vicinity. This MVA liquid crystal display device is an active matrix type liquid crystal display device in which each pixel is provided with a thin-film transistor (TFT) 102 as a switching element.
Gate bus lines 104 extending in the right-left direction in FIG. 21 and drain bus lines 106 extending in the top-bottom direction in FIG. 21 are formed on an array substrate 122 that is provided with the TFTs 102. Each TFT 102 is constructed of a drain electrode 108 that extends from the drain bus line 106, a source electrode 110 that is opposed to the drain electrode 108, and a portion (gate electrode) of the gate bus line 104 which overlaps with the drain electrode 108 and the source electrode 110. Although not shown in FIG. 21, channel layers made of, for example, an amorphous silicon (α-Si) film, are formed on the respective gate bus lines 104. The pixel electrodes 112 that are connected to the respective source electrodes 110 are further formed on the array substrate 122. Each pixel electrode 112 is provided with slits 114 that are oblique with respect to the edges of the pixel electrode 112. The slits 114 are alignment regulating structures on the array substrate 122 side for controlling the liquid crystal alignment. Each pixel electrode 112 is provided with connecting portions 116 so as not to be separated electrically by the slits 114. Thus, the pixel electrode 112 in each pixel is electrically connected.
On a common electrode substrate 128 that is provided with a common electrode and color filters (both not shown in FIG. 21) is formed a light shield film 136 (indicated by hatching in the top-left-to-bottom-right direction) in the regions where the TFTs 102 on the array substrate 122 are formed and other regions where neither the pixel electrodes 112 nor the alignment regulating structures (slits 114 in FIG. 21) are formed. The light shield film 136 is formed to suppress leak current that is caused by light incident on the channel layer of each TFT 102, prevent leakage of light from the adjacent pixel electrodes 112, and prevent color mixing between the adjacent pixels. For these reasons, the light shield film 136 is formed in such a manner that the edges approximately coincide with the edges of the pixel electrodes 112 when viewed in the direction perpendicular to the surfaces of the common electrode substrate 128. On the common electrode substrate 128 is also formed protrusions 118, which are the alignment regulating structures together with the slits 114 that are formed on the opposite array substrate 122.
For example, in the case of an XGA liquid crystal display device (LCD panel) having a diagonal size of 15 inches, each pixel measures 99 μm×297 μm. The width of the slits 114 and the protrusions 118 is 10 μm, and the interval between the slits 114 and the protrusions 118 is 25 μm when viewed in the direction parallel with the substrate surfaces. Further, the width of the connecting portions 116 of the pixel electrodes 112 is 4 μm, and the distance between the end portions of the drain bus lines 106 and the edges of the pixel electrodes 112 is 7 μm.
FIGS. 22 to 24, which are simplified sectional views taken along line E—E in FIG. 21, show functions of the slits 114 and the protrusions 118 that are the alignment regulating structures for controlling the liquid crystal alignment. FIG. 22 shows a state of the liquid crystal when no voltage is applied between the pair of substrates 122 and 128. In the array substrate 112 side, the pixel electrodes 112 are formed on a glass substrate 120, and the slits 114 are formed on the pixel electrode 112. Moreover, an alignment film (vertical alignment film) 126 is formed so as to cover the pixel electrodes 112 and the slits 114. On the other hand, in the common electrode substrate 128 side, the common electrode 124 is formed on the entire surface of the glass substrate 120 so as to be opposed to the pixel electrodes 112. The protrusions 118 made of an insulator (dielectric) such as a resist are formed on the common electrode 124. Moreover, an alignment film 126 is formed so as to cover the common electrode 124 and the protrusions 118.
A liquid crystal LC is sealed between the array substrate 122 and the common electrode substrate 128. Liquid crystal molecules (indicated by ellipses in FIG. 22) are aligned perpendicular to the alignment films 126. Therefore, the liquid crystal molecules are also aligned perpendicular to the alignment films 126 which are formed on the surfaces of the protrusions 118, and the liquid crystal molecules in the vicinity of the surfaces of the protrusions 118 are inclined against the glass substrate 120. However, strictly, the liquid crystal molecules in the vicinity of the protrusions 118 are not aligned perpendicular to the alignment films 126. In the regions where the protrusions 118 are not formed, the liquid crystal molecules are aligned approximately perpendicular to the glass substrates 120 by the alignment films 126. Because of the continuity of the liquid crystal, the liquid crystal molecules in the vicinity of the surface of each protrusion 118 follow the liquid crystal molecules located in the major part of the pixel, and hence, are inclined from the direction perpendicular to the alignment film 126 toward the normal to the glass substrate 120. Although not shown in FIG. 22, a pair of polarizers are disposed outside the glass substrates 120 of the array substrate 122 and of the common electrode substrate 128 in the crossed Nicols state. Therefore, black display is obtained when no voltage is applied.
FIG. 23 shows equipotential lines when voltages are applied between the electrodes of the pair of substrates. FIG. 24 shows a state of the liquid crystal in this condition. As indicated by equipotential lines (broken lines in FIG. 23), when voltages are applied between the pixel electrodes 112 and the common electrode 124, electric field distributions in the regions where the slits 114 or the protrusions 118 are formed differ from those in the other regions. This is because in each region where the slit 114 is formed, oblique electric fields are formed toward the common electrode 124 opposed from the end portions of the pixel electrode 112, and in each region where the protrusion 118 is formed, the electric field is distorted because the protrusion 118 is a dielectric formed on the common electrode 124. Therefore, as shown in FIG. 24, liquid crystal molecules fall in directions indicated by arrows in FIG. 24, that is, in such directions as to become perpendicular to the electric field directions, in accordance with the magnitude of the electric field. White display is thus obtained when voltages are applied.
Where the linear protrusions 118 are formed in a linear state as shown in FIG. 21, the liquid crystal molecules in the vicinity of each protrusion 118 fall in two directions that are approximately perpendicular to the direction where the protrusion 118 is provided, with the protrusion 118 being as the boundary. Since liquid crystal molecules in the vicinity of each protrusion 118 are slightly inclined from the direction perpendicular to the glass substrate 120 even when no voltage is applied, they fall quickly in response to the electric field. Nearby the liquid crystal molecules also fall quickly following the behavior of the liquid crystal molecules in the vicinity of the protrusion 118 while being influenced by the electric field. Similarly, where the slits 114 are provided in a linear state as shown in FIG. 21, the liquid crystal molecules in the vicinity of each slit 114 fall in two directions that are approximately perpendicular to the direction where the slit 114 is provided, with the slit 114 being as the boundary.
In this manner, in the region between two dash and dotted lines in FIG. 22, the liquid crystal molecules fall in the same direction, that is, they are aligned in the same direction. This is a region that is denoted by symbol [A] in FIG. 21. As denoted by symbols [A] to [D] in FIG. 21 in a typified manner, in the MVA liquid crystal display device, the four regions having different alignment directions are formed in each pixel, whereby a feature of a wide viewing angle is obtained. The above alignment control using the alignment regulating structures is not limited to the case of the combinations of the slits 114 and the protrusions 118 shown in FIGS. 21 to 24; a similar alignment control can be performed by using, as the alignment regulating structures, the combination of protrusions and protrusions or the combination of slits and slits.
Although the MVA liquid crystal display device provides a wide viewing angle, it had a problem that there exist regions where the alignment of liquid crystal molecules is not stable, resulting in lowering the luminance. That is, when voltages are applied between the electrodes, alignment defective regions 130 occur as hatched in FIG. 21. The alignment defective regions 130, where the light transmittance is low, are a factor of lowering the luminance in white display. The alignment defective regions 130 occur along the drain bus lines 106 on the side where the alignment regulating structures (protrusions 118 in FIG. 21) that are provided on the common electrode substrate 128 form obtuse angles with the edges of each pixel electrode 112 when viewed in the direction perpendicular to the substrate surfaces. In the alignment defective regions 130, the liquid crystal molecules have different alignment directions from the alignment directions that are controlled by the alignment regulating structures (protrusions 118 and slits 114 in FIG. 21) that are provided in the pair of substrates.
FIG. 25 is a plan view showing an MVA liquid crystal display device that solves the above problem and, more specifically, shows one pixel and a region in its vicinity. The components in FIG. 25 having the same functions as the corresponding components in FIG. 21 are given the same reference symbols as the latter and will not be described below. The MVA liquid crystal display device of FIG. 25 has auxiliary protrusions 132 that are alignment regulating structures for performing a strong alignment control in the alignment defective regions 130 shown in FIG. 21. The auxiliary protrusions 132 branch off the protrusions 118 and are formed along the end portions of each pixel electrode 112, that is, the drain bus lines 106. By virtue of the auxiliary protrusions 132, liquid crystal molecules (indicated by cylinders in FIG. 25) a are aligned continuously with liquid crystal molecules b, whereby the alignment of liquid crystal molecules is made stable in the alignment defective regions 130.
FIG. 26 shows a display area in which a band-like black figure that is long in the top-bottom direction in FIG. 26 (hereinafter referred to as “black vertical band”) is displayed against a white background on the conventional MVA liquid crystal display device of FIG. 25. In a display area 134, pixel A that is part of the background displays white, and pixel B in the black vertical band displays black. Although pixel C that is located under the black vertical band in FIG. 26 displays white, it is darker than pixel A. Pixel C becomes more darker than pixel A as the black vertical band becomes longer in the vertical direction in FIG. 26. As described above, the conventional MVA liquid crystal display device of FIG. 25 has the problem that when a black vertical band is displayed against a white background, a region displaying white under the black vertical band is displayed darker than the other region displaying white.
Similarly, FIG. 27 shows a display area in which a band-like white figure that is long in the top-bottom direction in FIG. 26 (hereinafter referred to as “white vertical band”) is displayed against a black background on the conventional MVA liquid crystal display device of FIG. 25. In the display area 134, pixel A that is part of the background displays black, and pixel B in the white vertical band displays white. Although pixel C that is located under the white vertical band in FIG. 26 displays black, it is brighter than pixel A. Pixel C becomes more brighter than pixel A as the white vertical band becomes longer in the vertical direction in FIG. 26. As described above, the conventional MVA liquid crystal display device of FIG. 25 has the problem that when a white vertical band is displayed against a black background, a region displaying black under the white vertical band is displayed brighter than the other region displaying black. In the following, the above-described phenomenon that pixel C and its vicinity become brighter or darker than pixel A and its vicinity will be referred to as “vertical crosstalk.” The vertical crosstalk occurs in such a manner that horizontal electric fields that develop between the drain bus lines 106 and end portions of each pixel electrode 112 where the auxiliary protrusions 132 are not formed influence the alignment of liquid crystal molecules.